Pressure transmitter

ABSTRACT

The invention relates to a pressure transmitter for an overload-proof pressure gauge, comprising a base body, a separating membrane disposed on a pressure-sensitive side of the pressure transmitter and subjectable to a pressure to be measured, a communicating chamber located inside said base body, which receives a pressure transmission medium, said chamber being closed on the pressure-sensitive side by the separating membrane, wherein the coefficient of thermal expansion of the pressure transmission medium is established in such a way that a temperature-induced change in the volume of the communicating chamber equals a temperature-induced change in the volume of the pressure transmission medium.

PRIORITY INFORMATION

This application is a continuation of Ser. No. 10/312,975 filed May 6,2003 now U.S. Pat. No. 7,062,974.

BACKGROUND OF THE INVENTION

The present invention relates to the field of pressure transducers, andin particular to the field of pressure transducers that include atemperature compensated pressure transmitter and a pressure sensor.

Typically, pressure-measuring devices consist of a pressure transmitterand a pressure sensor coupled with the pressure transmitter. Thepressure transmitter is subjected externally to a pressure, which iscoupled to and measured by the pressure sensor. The pressure transmitteroften serves to protect the extremely sensitive pressure sensor. Thepressure transmitter includes a separating membrane that seals off thepressure transmitter, and which together with a base body of thepressure transmitter defines a chamber that contains apressure-transmitting medium. The materials of the base body and theseparating membrane that together form the chamber typically havesignificantly lower coefficients of thermal expansion than thepressure-transmitting medium. Because of this difference, temperaturechanges in the environment lead to differing expansions in the basebody, the separating membrane, and the pressure-transmitting medium.This often results in relatively large measurement errors.

To reduce these temperature-induced measurement errors, pressuretransmitters have been designed that have a separating membrane with acomparatively large surface area on the process side. Alternatively oradditionally, the volume of the pressure-transmitting medium in thechamber is reduced. However, the volume of the pressure-transmittingmedium cannot be reduced at will because of the desired measuring rangeof the pressure sensor or its measurement tolerances. Moreover, apressure transmitter with a very small separating membrane diameter isoften required, since space considerations alone often place limits onthe size of the separating membrane in terms of area.

GM 76 03 126 describes a pressure transmitter in which an equalizingpart with a minimal expansion coefficient is built into the chamber ofthe base body of the pressure transmitter to compensate for the volumeexpansion of the pressure-transmitting medium. However, such a pressuretransmitter is extremely difficult to produce. In addition, adjustingthis temperature-compensated pressure transmitter to provide the exactsetting of the proportions of the pressure-transmitting medium and theequalizing body is extremely expensive. Finally, particularly withpressure gauges for measuring pressures in the millibar range, it isnecessary for the pressure transmitter to transmit the pressure receivedfrom the outside reliably and evenly to the pressure sensor downstreamthereof. However, with the pressure transmitter described in GM 76 03126, in which the equalizing part floats freely in the chamber, this isnot possible or is possible only to a limited extent.

Therefore, there is a need for a temperature compensated pressuretransmitter for use in a pressure transducer.

SUMMARY OF THE INVENTION

Briefly, according to an aspect of the present invention, a pressuretransmitter includes a base body having a peripheral wall, and aseparating membrane attached to the separating wall to form a chamberbetween the base body and the membrane. A pressure transmitting mediumis contained within the chamber. The coefficients of thermal expansionof the separating membrane and of the base body, and the coefficient ofthermal expansion of the pressure-transmitting medium are such that atemperature-induced volume change in the chamber is at leastapproximately the same as a temperature-induced volume change in thepressure-transmitting medium.

Advantageously, optimal temperature compensation of the pressuretransmitter and hence of the pressure gauge is achieved in a simplemanner. An advantage of the pressure transmitter according to theinvention is that the pressure transmitter head does not have to beenlarged to produce the temperature compensation. Moreover, noadditional expensive manufacturing process steps are required. Once apressure transmitter is specified—namely over the appropriate selectionof coefficients of thermal expansion, the compensating volume of theseparating membrane and the volume increase in the pressure-transmittingmedium offset each other—no additional manufacturing steps are necessarywhen manufacturing the pressure transmitter.

The separating membrane may have a corrugated shape of the radiallyouter areas of the separating membrane. As the temperature increases,the corrugation of the separating membrane decreases, leading to anincrease in the volume of the chamber—the so-called compensating volume.The corrugation allows the compensation to be intentionally set for agiven temperature-induced increase in volume.

The head of the pressure transmitter may have a round cross section ofthe separating membrane. Of course, separating membranes with othershapes for the pressure transmitter head can also be made, for exampleoval, hexagonal, or square, but the round shape is by far the mostaccurate, especially for pressure measurement in the millibar range.Accordingly, advantageous fully compensated pressure transmitters can beproduced whose separating membranes have a diameter of less than 40 mm.

Typically, the base body and the separating membrane are made at leastpartially of a corrosion-resistant metal material. High-grade steel ispreferably used. It would be possible, however, for the separatingmembrane and/or the base body to be made of a ceramic or ceramic-likematerial. An oil, for example a hydraulic oil or silicone oil, istypically used as the pressure-transmitting medium. However, any otherliquid or even a gas may be used.

The base body and the separating membrane typically have coefficients ofthermal expansion that are far smaller than the pressure-transmittingmedium. Usually the coefficient of thermal expansion is in the range of10-100×10⁻⁶/K for a temperature range of 20-100° C. and is hence smallerby a factor of 10 to 1000 than that of the pressure-transmitting medium.

These and other objects, features and advantages of the presentinvention will become more apparent in light of the following detaileddescription of preferred embodiments thereof, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial section view of a first embodiment of a pressuretransmitter; and

FIG. 2 is a partial section view of a second embodiment of a pressuretransmitter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a partial section through a pressure transmitter 1, of apressure gauge. The pressure transmitter 1 includes a metal pressuretransmitter base body 2, which preferably includes a corrosion-proofoxidation-resistant material. However, the base body 2 may include atleast partially another material such as for example a ceramic material.On a sensor side 3, the base body 2 has a flange 4 that connects thepressure transmitter 1 with a pressure sensor (not shown).

On a process side 5, the pressure transmitter 1 has a separatingmembrane 6 disposed in a recess 7 formed in base body 2. The membrane 6is disposed in the recess 7 in such a way that a chamber 8 is formedbetween the membrane 6 and the base body 2. In addition, the base body 2of the pressure transmitter 1 has a hole 9 connected with the chamber 8and with the flange 4. A pressure-transmitting medium (e.g., an oil) islocated in the communicating chamber 8 and the hole 9. The membrane 6separates the medium on the process side (not shown in the figure) fromthe pressure-transmitting medium inside the communicating chamber 8. Apressure applied to the process side is transmitted to thepressure-transmitting medium via the separating membrane 6, and hence tothe pressure sensor downstream.

The head of the pressure transmitter 1, and hence the membrane 6, areessentially circular in cross section in the present example. Themembrane 6 has a corrugated shape at least in the radially outer areasof the separating membrane 6.

The base body 2 of the pressure transmitter 1 and the separatingmembrane 6 include material that typically has a much lower coefficientof thermal expansion than the pressure-transmitting medium within thechamber 8. With fluctuating temperatures, the membrane 6 is compressedor stretched radially. If the temperature increases for example fromT=20° C. to T=100° C., the membrane 6 is stretched (see dashed line inFIG. 1) so that the volume of the chamber 8 increases. However, if thetemperature decreases, the membrane 6 is compressed, reducing the volumeof the chamber 8. A compensating volume 10 results from the volumedifference between the compressed and the stretched membrane 6. When thetemperatures increase and decrease, the pressure-transmitting mediumincreases and decreases in volume, and so does the above-mentioned oilfill.

According to an aspect of the invention, the total volume of thepressure-transmitting medium as well as the coefficients of thermalexpansion of the materials of which the base body 2 and the membrane 6are adjusted such that the temperature-induced change in the volume ofthe pressure-transmitting medium is the same, or at least approximatelythe same, as a temperature-induced change in the compensating volume 10.As a result, the pressure transmitter 1 is temperature-compensated overwide ranges. That is, the closed hydraulic or pneumatic system of thepressure transmitter 1 is temperature-compensated due to correspondingincreases in volume of the pressure-transmitting medium and thecommunicating chamber 8, 9.

The temperature dependence of the pressure transmitter 1 can be reducedto nearly zero by the following measures, which can be taken even invery small measuring ranges of approximately 100 mbar. First, thetemperature dependence can be adjusted by an appropriate materialcombination of the base body 2 and the separating membrane 6, forexample by appropriately choosing their thermal expansion coefficients.Alternatively or additionally, the temperature dependence of thepressure transmitter 1 can be further reduced by a suitable shape of theseparating membrane 6.

In addition, it is especially advantageous for the separating membrane 6to be concave for example at a low temperature (e.g., T=20° C.) andassume an increasingly convex shape at increasing temperatures (e.g.,T=100° C.). Of course, all other measures for reducing the temperaturedependence are possible and advantageous, for example reducing thevolume of the pressure-transmitting medium, enlarging the surface areaof the separating membrane 6, and reducing the distance between thepressure transmitter 1 and the downstream pressure sensor as far aspossible. Thus, in broad ranges, optimum temperature compensation of thepressure transmitter and hence of the pressure gauge can be achieved bythese measures.

FIG. 2 is a section through a pressure transmitter 100. The embodimentillustrated in FIG. 2 is substantially the same as the embodiment inFIG. 1, with the principal exception that the pressure transmitterincludes a separating membrane 106 that is not corrugated in its outerarea, but is straight. Overall, the separating membrane 106, shown onceagain at T=20° C. (solid line) and at T=100° C. (dashed line) has theshape of a pot.

Although the present invention has been shown and described with respectto several preferred embodiments thereof, various changes, omissions andadditions to the form and detail thereof, may be made therein, withoutdeparting from the spirit and scope of the invention.

1. A pressure transmitter, comprising: a base body having sidewalls thatform a recess; a membrane that seals the recess to form a chamber incooperation with the base body; a pressure transmitting medium withinthe chamber, where the pressure transmitting medium is acted upon by anexternal pressure on an exterior side of the membrane; wherecoefficients of thermal expansion of the membrane and of the base bodyand the coefficient of thermal expansion of the pressure transmittingmedium are established such that a temperature-induced volume change inthe chamber is approximately the same as a temperature-induced volumechange in the pressure transmitting medium.
 2. The pressure transmitterof claim 1, where at least part of the membrane has a corrugated shape.3. The pressure transmitter of claim 2, where the membrane is partiallypot shaped.
 4. The pressure transmitter of claim 2, where the membranehas a round cross section with a diameter of less than 40 mm.
 5. Thepressure transmitter of claim 1, where the base body comprises acorrosion-resistant metal.
 6. The pressure transmitter of claim 1, wherethe pressure transmitting medium contains an oil.
 7. The pressuretransmitter of claim 1, where the base body has a temperature expansioncoefficient that is smaller by at least a factor of 10 than that of thepressure transmitting medium.
 8. The pressure transmitter of claim 1,where the base body (has a temperature expansion coefficient in therange of 10-100×10⁻⁶/K for a temperature range of 20-100° C.
 9. Apressure transmitter for use in a pressure transducer, said pressuretransmitter comprising: a base body; a membrane attached to the basebody to form a chamber between said base body and said membrane; and apressure transmitting medium contained within said chamber; whereincoefficients of thermal expansion of said membrane and said base body,and the coefficient of thermal expansion of said pressure transmittingmedium are such that a temperature-induced volume change in said chamberis at least approximately the same as a temperature-induced volumechange in said pressure transmitting medium.
 10. The pressuretransmitter of claim 9, where the membrane comprises a corrugatedsurface that forms a surface of the chamber.
 11. The pressuretransmitter of claim 9, where the pressure transmitting medium comprisesa liquid.
 12. The pressure transmitter of claim 11, where the liquidcomprises an oil.
 13. The pressure transmitter of claim 12, wherein themembrane comprises a ceramic.
 14. A pressure transmitter, comprising: abase body having a peripheral surface; a membrane attached to the basebody to form a chamber between said base body and said membrane; and amedium contained within said chamber; wherein coefficients of thermalexpansion of said membrane, said base body, and said medium are suchthat a temperature-induced volume change in said chamber is at leastapproximately the same as a temperature-induced volume change in saidmedium.
 15. The pressure transmitter of claim 14, where the membranecomprises a corrugated surface that forms a surface of the chamber. 16.The pressure transmitter of claim 14, where the pressure transmittingmedium comprises a liquid.
 17. The pressure transmitter of claim 16,where the liquid comprises oil.
 18. The pressure transmitter of claim16, wherein the membrane comprises a ceramic.